1. Field of the Invention
This invention relates to the continuous casting industry. More specifically, this invention relates to an improved guide segment support system and method for supporting and guiding a cast strand after it emerges from a mold.
2. Description of the Related Technology
Metals such as steel are continuously cast into strands by pouring hot, molten metal into the upper end of a mold and continuously withdrawing a metal strand from the mold's bottom. As the molten metal passes through the mold, the surfaces of the metal that are adjacent to the mold walls are cooled, solidified and hardened to form a casing or shell of solidified metal around the molten metal in the strand. After leaving the bottom of the mold, the metal continues to cool and the casing or shell of solidified metal around the molten core thickens until the whole strand section is solidified.
The shell of solidified metal around the molten core, as the continuous cast strand leaves the mold, is relatively thin and fragile, and requires support. Such support, in continuous casting of metals, is customarily provided by rolls which engage and support the opposite sides of the continuously cast strand. The supporting rolls immediately below the mold, where the shell of solidified metal is relatively thin, are usually of relatively small diameter and are longitudinally spaced closely together. To assist cooling of the slab and to prevent the rolls and bearings from overheating, these supporting rolls may be liquid cooled. Further away from the mold bottom, where the metal has cooled and the shell of solid metal has thickened, rolls of larger diameter, spaced at greater longitudinal distance, are usually employed. To control the casting speed, certain of the supporting and guiding rolls may be driven. Typically, of course, the supporting rolls are arranged about an arcuate path or apron that defines the path of the strand as it emerges vertically downwardly from the mold, then gradually bends about a 90 degree arc until it emerges as a fully solidified, horizontally oriented casting. Space is generally provided between the rolls for permitting introduction of spray water to cool the cast strand.
Conventionally, the opposed supporting and guiding rolls are divided into segments. On the outside radius of the arcuate path that is defined by the guide rollers, a supporting frame (commonly referred to as a "banana beam") is provided to which these segments are attached. In order to permit repair and maintenance work to be effected on the segments, the segments are designed to be exchangeable. The segments of the apron can be exchanged with the help of a segment changing carriage which runs on rails extending parallel to the apron, or, in some machines, can be lifted out by a building-mounted crane system. The segments can be transferred to this carriage and carried thereon to a point where they can be removed from the plant and repaired or readjusted as may be required.
Each segment includes a "bottom" portion containing a bottom set of rolls that engages the side of the strand that is closest to the supporting frame or banana beam, which is the side that defines the outer radius of the arc through which the strand is guided to move. The bottom portion of the segment is always, in the experience of the inventor, securely mounted to the supporting frame so that no relative movement is permitted between the bottom portion and the supporting frame. Each segment will also include a "top" portion that holds a top set of rolls for supporting the inner radius side of the strand. To provide the necessary support to the strand, and to counter ferrostatic pressure that develops in the strand during its vertical descent, the top portion and the bottom portion are urged together by a controlled force, which is typically exerted by a hydraulic mechanism that is mounted on the segment. The top and bottom portion alternatively are connected by a spring structure jacking mechanism that permits limited movement therebetween during operation.
During normal casting operation, the top and bottom portions of the strand are urged together at a relatively constant force against mechanical stops so that the distance between the opposing top and bottom rolls is maintained constant. Unfortunately, the magnitude of this force can be so great that damage to the rolls, roll bearings, roll supports and segments can and often do occur, particularly during abnormal casting conditions. For example, if the casting machine is caused to stop for any length of time, it might be necessary to withdraw the solidified strand from the apron of support rollers by performing what is referred to in the industry as a "cold strand withdrawal." In this procedure, the fully solidified strand is caused to be bent into its desired final horizontal shape as it travels through the arc-shaped array of guide and support rolls by the pressure that is exerted on the strand by the rolls. In current systems, this often initiates failure of one or more guide rolls (usually, in the inventor's experience, by yielding or fracturing the bearing races), although it might take several weeks to find out which rolls are going to fail. Bearing failure, in the inventor's experience, most often occurs in the bottom portion of the segment, which is rigidly mounted to the banana beam. Of course, when failure does occur, it adversely affects the quality of the cast product, which is a major economic concern for the steel maker. Other conditions that can cause failure of the rolls include bulges or irregularities in the strand being cast. If the irregularity is on the top side of the strand, forces may be partially absorbed by the spring structure or by the opening of the hydraulic clamping cylinders by action of associated pressure relief valves. If the irregularity is on the bottom, though, the spring structure does not provide much protection against a force overload condition being applied against the lower rolls.
A need exists for an improved strand support segment system that is designed so as to minimize the potential for early failure as a result of abnormal conditions such as cold strand withdrawal or the presence of irregularities in the strand being cast.